Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Smack => Topic started by: Jim R. on May 22, 2012, 06:48:04 AM

Title: Help needed sorting out the balanced/unbalanced connections and switching.
Post by: Jim R. on May 22, 2012, 06:48:04 AM
Hoping one of the BH guys can help here...

I want to build my Smack with both balanced and unbalanced outputs -- though not quite as the standard chassis is layed out.  For each kind of output I want two connectors -- one larger and one 3.5mm -- so a standard TRS and a mini TRS for the unbalanced side, and a mini 4-pin and a 3.5mm TRRS for the balanced side.  Then I want to machine the top plate where the existing XLR connectors are and replace it with a small plate with 3 switches on it for the bal/unbal and impedance switching.

II'm having great difficulty trying to decipher the switching arrangement from the manual, and I would like some guidance with the following things:

1) are balanced and unbalanced outputs wired to separate sets of terminals on these switches (or the switch PCBs if I were going that route)?  I recall PJ saying something about plugging a balanced set of cans into unbalanced jack will result in mono sound, which again I still don't fully understand, but why I'm assuming that separate wiring for balanced/unbalanced connectors need to be made.  Is this correct?

2) Wondering if PJ can either post or email me the net list of connections from the OPTs, directly to switches and jacks?  I recall Doc once saying that there are 2 DPDT switches and one SPDT on the PCBs, in which case I'd just get some 4pdt and a spdt and wire each half of the switch such that one switch, accessible on the top panel, will change both channels simultaneously.  I haven't looked at the parts bags yet, but in the manual I believe it says there are 3 spdt switches, so some help sorting out the switch types would be helpful too.

Living situation is about to change drastically (for the better), but my bedroom system will be confined to one headphone amp and a source, and I want to be using the most flexible amp for both regular cans and IEMs, both balanced and unbalanced (yes, I have all variations of the above.)


Title: Re: Help needed sorting out the balanced/unbalanced connections and switching.
Post by: Paul Joppa on May 22, 2012, 10:32:07 AM
You'd think it would be simple - only three switches and some wire. Bu it took me several days (and hour or two each day) before I got it into q circuit diagram, and about the same time to get the PC board drawn up. For some reason, I do tend to get confused when looking at switch wiring.

Anyhow, I'll start with a description:

There are two secondary windings on the transformer, each tapped at the 2/3 point - so six terminals connect to the PC board. There are three outputs from the board, positive, negative, and ground. The board has three identical DPDT switches, one of which is wired in parallel so it's really a SPDT. And there are two resistors.

Switch 1 chooses whether the taps at 2/3 of the winding are used, or the full winding.

Switch 2 chooses whether the windings are in series or in parallel. Both of these are DPDT. By a tricky arrangement, switch 2 also inserts a resistor divider between positive and negative if the windings are in parallel. The divider makes a virtual centertap for balanced out.

Switch 3 - the SPDT one - chooses whether the negative or the centertap goes to ground. Again, some tricky wiring makes it so that the ground goes to the center of the resistor divider if the windings are in parallel, or to the center connection if they are in series. The resistors are removed from the circuit in the series wiring scheme.

The positive output goes to pin 2 of the XLR for balanced wiring, and to the tip or ring for unbalanced output. The negative output goes to pin 3 of the XLR. The ground goes to the shield or common for unbalanced outputs, and to pin 1 for the XLR. In normal use, the ground output of the switch board also goes to chassis ground.

I hope that provides a useful overview.

I'm sorry I don't have time today to convert the circuit to words; I'll try to get to that in a day or two.
Title: Re: Help needed sorting out the balanced/unbalanced connections and switching.
Post by: Jim R. on May 22, 2012, 10:50:22 AM
Hi Paul,

Excellent -- thanks for the background -- really helpful, and answers many questions.

So, I can relate to switch wiring confusion -- even when I had my sight I too had to do these things with great care and a lot of frustration -- so you can probably imagine how much fun it is now :-).

No rush on the net list, but that would be supremely helpful for when I go to build the amp.

Thanks again,